1. Field of the Invention
This invention relates generally to chemical-mechanical planarization or polishing (CMP) tools and, more particularly, to a system and method, which combine work-piece film measurements with a spin-dry process.
2. Description of the Related Art
Briefly, the chemical mechanical polishing process requires that a workpiece be held, with the desired coated surface face down, on a polishing pad supported on a rotating table, in the presence of an abrasive slurry. A chemical mechanical polishing machine (unit) can include a single rotating polishing plate and a smaller diameter rotating wafer carrier to which a wafer (or wafers) is (are) mounted. The wafer carrier is held above the polishing plate, in either a stationary fixed position or oscillating back and forth in a predetermined path, while both polishing plate and wafer carrier are rotated about their respective center axes. A slurry, consisting of an abrasive suspension with or without an etching reagent, is fed onto the polishing plate during polishing of the wafer. The slurry, also referred to as a carrier liquid, can be selected to include an etchant for the coating being planarized and for not substantially attacking other materials involved in the process. The slurry is further fed between the polishing plates to polish and flush away the material removed from the semiconductor wafer. Current CMP tools are built with a constrained processing sequence whereby wafers are loaded from a cassette, polished, rinsed, cleaned, dried and unloaded. In some cases there are multiple polish steps where a wafer is polished first in one polishing medium, rinsed, polished in a second medium, rinsed, cleaned, dried and unloaded.
In particular, when using various CMP polishing tools in processing a wafer where a sufficient amount of surface films has been polished (removed by grinding), the wafer is rinsed and cleaned to remove slurry and then dried in a spin-dry process using a CMP spin-dry tool. Further, when a CMP process polishes transparent materials such as oxides and nitrides in addition to metals, the process is typically controlled by performing an optical film thickness measurement. For example, back end silicon dioxide layers are typically polished from a starting thickness of 20000 angstroms to a final thickness of 10000 angstroms. To control the process, wafers have to be removed periodically from the spin dryer and measured on an optical film thickness measurement tool that examines the film reflectivity spectrum across a range of optical wavelengths to determine the thickness of material from the optical fringe pattern. Thickness measurements are typically made at many points on the wafer (usually six or more). The deviation of this thickness value from a desired value is used to adjust the polish time for subsequent wafers up or down to assure that they are polished to the correct thickness.
The measurement of thin films on semiconductor and other micro-manufactured parts is typically performed by an optical interference technique in which the reflectance or transmission properties are measured using an optical probe using a separate apparatus. Then, the acquired spectrum is analyzed with a computer program referred to as a "recipe" using known film properties and physics to solve for unknown properties such as film thickness, density etc. For ideal films having one or more perfectly flat layers, these measurements are straight forward and well documented.
However, prior to the invention, there has been no effective structure or operation that minimizes the number of steps or machines used to perform this process, which is slow and expensive since wafers being processed must be manually loaded into a stand alone optical film thickness measurement tool. This represents significant delay between the time the wafer is polished and the time the film thickness measurement is completed. Where precise control is essential, the process is halted until the thickness measurement results are known. This causes both the introduction of additional errors due to tool drift during idle time and cost in terms of process cycle time.
In addition, known processes have had to stop processing and determine variations of thickness of a wafer being processed to determine whether the CMP polishing unit is operating within acceptable tolerances to control basic thickness of the resultant polished films on the wafer. When the CMP polishing unit is in need of maintenance and/or out of tolerance, this state of operation incurs processing error, thus additional unwanted processing expense. This is generally caused by the CMP polishing pads wearing out and in need of replacement, and the polishing tool often needs to be calibrated and adjusted. Mechanical part wear and failure is evident when examining the thickness distribution of a polished wafer. The normal polish distribution on a wafer is a radially symmetric distribution which is typically low in variation magnitude. When the range or distribution of thickness values becomes unusually large, this is interpreted as need for maintenance, which is slow and expensive.